1. Field of the Invention
The present invention relates to the construction of a distributed amplifier and differential distributed amplifier for use in high-frequency amplifiers in the microwave band, in the millimeter wave band, etc.
2. Description of the Related Art
Generally, when amplification is performed by using FETs, it is required to increase the total gate width of the FETs in order to obtain higher output. When the frequency to be used is high, even if the unit gate width is increased, this measure is ineffective because of electrical delay, gate resistance loss, etc., and accordingly a comb-shaped construction with a large number of gate fingers is adopted.
The construction of a known type of comb-shaped FET is shown in FIG. 9. As shown in FIG. 9, the comb-shaped FET 80 is made of a unit FET 81a which is composed of a source electrode 84, a drain electrodes 86, and a gate electrode 85 disposed therebetween. A neighboring unit FET 81b has the same drain electrode 86 as the unit FET 81a. In this way, six unit FETs are constructed. These unit FETs are disposed so as to have a comb-shaped construction by using a source wiring 87, a drain wiring 82, and a gate wiring 88. Moreover, the source wiring 87 is connected to grounding portions (source pads) 83a and 83b, the drain wiring 82 is connected to an output portion (drain pad) 90, and the gate wiring 88 is connected to an input portion (gate pad) 89. Here, the unit gate width of the unit FET is shown by 85T.
The circuit diagram of a distributed amplifier using the comb-shaped FET of FIG. 9 is shown in FIG. 10. As shown in FIG. 10, in the distributed amplifier 100, a plurality of unit FETs 112 each made of a source electrode 109, a drain electrode 111, and a gate electrode 110 are connected in parallel. An input portion 107 comprises an input terminal connected to the gate electrode of each unit FET through high-impedance transmission lines 105. The opposite end of the input portion 107 is grounded through a terminating circuit made of a resistor 101 and a capacitor 103. Furthermore, an output portion 108 comprises an output terminal connected to the drain electrode of each unit FET through high-impedance transmission lines 106. The opposite end of the output portion 108 is grounded through a terminating circuit made of a resistor 102 and a capacitor 104. Furthermore, the source electrode of each unit FET is grounded.
However, in the comb-shaped FET 80, the lengths of the paths in which a signal passes inside the FET 80 are short in the unit FETs located close to the input portion 89 and the output portion 90, that is, in the inside unit FETs, and the lengths of the paths are long in the outside unit FETs. Therefore, because of the difference in the paths in which a signal passes, when a signal is distributed to each gate electrode, a difference in phase of the signals is caused, and also there is a problem in that the phase difference is produced when the signals from all the drain electrodes are synthesized. Furthermore, when each source electrode is connected to the grounding portions 83a and 83b, since the line lengths, which are dependent on the location of each unit FET, are different from each other, there is a problem in that the source impedance is different from one unit FET to another.
In order to avoid such effects, as the frequency to be used increases, the size of the FETs is required to be reduced, but this is contradictory to the purpose of obtaining higher output. Therefore, there is a problem that it is difficult to increase the output even if a comb-shaped construction is adopted.
Moreover, in the distributed amplifier 100 using this comb-shaped FET, a resistor 101 is used in the terminal of the circuit which electrically connects the gate electrodes in consecutive order. Because of this, in low-noise amplifiers, there is a problem in that the NF (noise figure) increases. Furthermore, a resistor 102 is used in the terminal of the circuit which electrically connects the drain electrodes in consecutive order. Accordingly, in high-output amplifiers, there is a problem in that the electric power is consumed in the resistor 102.
In order to overcome the problems described above, preferred embodiments of the present invention provide a distributed amplifier and differential distributed amplifier in which a low noise amplifier or high output amplifier can be easily fulfilled without causing phase differences.
According to a preferred embodiment of the present invention, a distributed amplifier may comprise two or more unit FETs, each having a source electrode, a drain electrode, and a belt-shaped gate electrode arranged therebetween, the unit FETs being substantially linearly arranged in the length direction of the gate electrode on a substrate; each pair of neighboring source electrodes, neighboring drain electrodes, and neighboring gate electrodes being electrically connected to each other by a respective circuit in consecutive order of the two or more unit FETs; and an input portion connected to one end of the circuit that connects the gate electrodes, and an output portion connected to one end of the circuit that connects the drain electrodes. In the distributed amplifier, an inductor or high-impedance transmission line may be provided in one or more of the connection portions in the circuits between the neighboring drain electrodes or between the neighboring gate electrodes; or a capacitor may be provided in one or more of the connection portions in the circuits between the drain electrodes and the source electrodes or between the gate electrodes and the source electrodes.
Furthermore, in a distributed amplifier of the present invention, a terminating circuit having any one of an inductor, a capacitor, and a transmission line or having a combination of these may be connected to the end opposite to the input portion of the circuit that connects the gate electrodes electrically in consecutive order.
Furthermore, in a distributed amplifier of the present invention, a terminating circuit having any one of an inductor, a capacitor, and a transmission line or having a combination of these may be connected to the end opposite to the output portion of the circuit that connects the drain electrodes electrically in consecutive order.
Furthermore, in a distributed amplifier of the present invention, a terminating circuit having any one of an inductor, a capacitor, and a transmission line or having a combination of these may be connected to the end opposite to the input portion of the circuit that connects the gate electrodes electrically in consecutive order and to the end opposite to the output portion of the circuit that connects the drain electrodes electrically in consecutive order.
Furthermore, in a distributed amplifier of the present invention, a terminating circuit having at least a resistor may be connected to the end opposite to the output portion of the circuit that connects the drain electrodes electrically connected in consecutive order.
Furthermore, in a distributed amplifier of the present invention, a terminating circuit having at least a resistor may be connected to the end opposite to the input portion of the circuit that connects the circuit gate electrodes electrically in consecutive order.
Furthermore, a differential distributed amplifier may comprise two of the distributed amplifiers described above; and a belt-shaped source wiring metal formed between the distributed amplifiers. In the differential distributed amplifier, the two distributed amplifiers are arranged symmetrically with respect to said source wiring metal with the respective source electrodes facing each other, and the source wiring metal interconnects the respective source electrodes of the pair of distributed amplifiers.
Furthermore, in the differential distributed amplifier, the terminating circuits connected to the ends opposite to the input portions of the respective circuits interconnecting the corresponding gate electrodes, are electrically separated from the source wiring metal, and are connected to each other.
Furthermore, in the differential distributed amplifier, the terminating circuits connected to the ends opposite to the output portions of the respective circuits interconnecting the corresponding drain electrodes, are electrically separated from the source wiring metal, and are connected to each other.
When constructed in this way, in a distributed amplifier and differential distributed amplifier of the present invention, low-noise amplification or high-output amplification can be easily fulfilled without having any phase difference.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.